1 AnIntroductiontoIonicLiquids · While much of the early work on ILs/molten salts was driven by interest in their use as electrolytes, the potential beneﬁt of these materials to

1

1

An Introduction to Ionic Liquids

1.1 Prologue

The benefits of using salts in their liquid form as electrolytes or reaction mediahave long been recognized. For example, Faraday developed his laws of electrol-ysis in the 1830s using molten metal halide salts. However, most researcherswould rather avoid using solvents that require heating to hundreds of degrees.Therefore, while ‘high-temperature molten salts’ are extremely useful for certainapplications, they are not in widespread use in research laboratories or industry.In contrast, ‘ionic liquids’ – and we will discuss the definition of these below– areproving to be very exciting for a very wide range of applications atmoremoderatetemperatures. Particularly over the last decade, scientists working inmany differ-ent areas of research have started to realize the special properties of ionic liquids(ILs) and to embrace the promise of these new materials. As more applicationsare discovered, and more families of ILs are developed, so the field continuesto grow. Thousands of papers on ILs are now published every year, and, moreimportantly, increasing numbers of researchers are experimenting with ILs anddiscovering firsthand how their unique properties can help their work.So here we are, just over 100 years since the first ‘room-temperature ionic

liquid’ was discovered, at a point where these materials are now proving sopromising and widely applicable that it is important for a broad range ofscientists and engineers to have an appreciation of the basics of ILs. Thus, thepurpose of this book is to serve as an introduction to the key concepts andapplications of ILs for those venturing into this field for the first time. Hopefully,the book will also inspire further curiosity and enthusiasm for exploring theseexciting and very unique materials.Our goal in this book is to provide a thorough introduction to the field

appropriate to the level of a finishing undergraduate science student or a begin-ning postgraduate student. Our emphasis is on illustrative examples and thebackground chemistry sufficient to understand the fundamentals of ILs and theirapplications. For further reading, we have referenced more extensive reviewswhere they exist. To provide background on fundamental concepts and methodsthat may not be readily accessible in standard textbooks, we have includedConcept Toolbox items as breakout text boxes in various places throughout thechapters. This first chapter provides a broad overview of the field, the materials

Fundamentals of Ionic Liquids: From Chemistry to Applications, First Edition.Doug MacFarlane, Mega Kar and Jennifer M Pringle.© 2017Wiley-VCH Verlag GmbH & Co. KGaA. Published 2017 byWiley-VCH Verlag GmbH & Co. KGaA.

2 1 An Introduction to Ionic Liquids

involved, their properties, and their applications, of which more details can befound later in this book.

1.2 The Definition of an Ionic Liquid

The phrase ‘ionic liquid’ was coined only relatively recently to refer to ambient-temperature liquid salts, and the definition has since been the subject of muchdiscussion and some evolution. The most useful practical definition of an IL is

‘A liquid comprised entirely of ions.’

We can delve into this a little deeper. By this definition, is an IL different from amolten salt?The answer is: ‘No’ – the term ‘molten salt’ refers to the liquid phaseof a crystalline salt, for example, NaCl. ‘Ionic Liquid’ covers that, but also coversa broader range of possibilities. Imagine a mixture of the two salts Na[fsi] and[C3mpyr][NTf2] (see Table 1.1 for an explanation of abbreviations). This mixtureof salts is a liquid at room temperature and is properly called an IL by our defini-tion. In fact an IL could contain a very large number of different ions. Note that,in common usage, the term ‘molten salt’ has also come to meanmixtures of salts,although the term itself clearly indicates a single compound.In principle (in fact an important thermodynamic principle), the IL obtained

by mixing Na[fsi] and [C3mpyr][NTf2] is exactly the same as that obtained bymixing the appropriate quantities of Na[NTf2] and [C3mpyr][fsi]. The points oforigin are irrelevant in defining the IL; only the quantities of the individual ionspresent are important. In fact, such ILs with very high concentrations of Na or Lisalts are proving to be highly effective as electrolytes for Na and Li batteries [1].Some definitions of IL add a temperature range, such as ‘below 100 ∘C’, to the

definition but this is not necessary. In fact, it is limiting to do so, since it canblinker our perspective on which compounds or mixtures may be useful for cer-tain applications. Indeed, there are many, quite valuable, applications of ILs attemperatures above 100 ∘C, for example, the preparation of MnOx water oxida-tion catalysts by electrodeposition at 130 ∘C [2]. The key requirement for thisis that the IL be a liquid at 130 ∘C. It is convenient if it is also liquid at roomtemperature, but it is not necessary for this to be the case and one should cer-tainly not exclude from consideration compounds havingmelting points>100 ∘Cfor an application such as this. Similarly, a definition that includes ‘a salt havinga melting point below 100 ∘C’ (or some other temperature such as room tem-perature) is also an unnecessary restriction because in some cases the meltingpoint may be practically difficult to find and measure. The supercooling of liq-uids below their equilibrium melting points is a well-known phenomenon, andin some cases the liquid becomes so viscous that the crystalline phase never formson a practical timescale. This is particularly true of mixtures of salts, which wehave agreed are perfectly good ILs, because themelting points of individual com-pounds is often sharply depressed inmixtures.Wewill discuss themelting pointsof ILs in Chapter 2, andmulticomponent phase diagrams and behavior further inChapter 5. With all of this in mind, referring to a melting point in our definitionof an IL becomes unhelpful.

1.2 The Definition of an Ionic Liquid 3

Table 1.1 Glossary of structures and nomenclature abbreviations used in this book.

Quaternary cations Abbreviation Structure

Alkylmethylimidazolium [Cnmim]+

N NR

+

Alkyldimethylimidazolium [Cndmim]+

N NR

+

Alkylmethylpyrrolidinium [Cnmpyr]+

N+

R

Alkylmethylpiperidinium [Cnmpip]+

N+

R

Ammonium [Nn,n,n,n]+

RN+

R

RR

Phosphonium [Pn,n,n,n]+

RP+

R

RR

Ether-functionalized [NR,R,R,2O1]+ , etc.

N+R

R

RO

Cholinium [Ch]+HO N+

Sulfonium [SR,R′ ,R′′ ]+

S+

R

R″ R′

Guanidinium [Gdm]+

C

+NH2

H2N NH2

(Continued)


Table 1.1 (Continued)

Anions Abbreviation Structure

Tetrafluoroborate [BF4]−

B–

F

F

F

F

Hexafluorophosphate [PF6]−

FP–

F

FF

FF

Trifluoroacetate [tfa]−

F3C

O

O–

Triflate [OTf]− or [CF3SO3]−

S

O

O

O–F3C

Bis(fluorosulfonyl)imidea) [fsi]− or [N(SO2F)2]−

SF

N–

SF

O

O

O

O

Bis(trifluoromethanesulfonyl)imidea) [NTf2]− or[N(SO2CF3)2]− S

O

ON–

F3C S

O

O

CF3

Dicyanamide [N(CN)2]− or [dca]−

N

N–

N

Tetracyanoborate [B(CN)4]−

B–

CN

CN

CN

NC

Fluoroalkylphosphates [fap]−, [efap]−, etc.C2H5

C2H5

C2H5

F

F

F

P–

Dihydrogenphosphate [H2PO4]− or [dhp]−

P

OH

O–

OH

O

1.2 The Definition of an Ionic Liquid 5

Table 1.1 (Continued)

Anions Abbreviation Structure

Hydrogen sulfate [HSO4]−

S

O

O–HO

O

p-Toluenesulfonate or tosylate [Tos]−SO3

–

Tetrahydroborate or borohydride [BH4]−

B–

H

H

H

H

a) Also referred to as ‘amides’.

The meaning of the word ‘ion’ in this definition also needs some discussion.Species such as Cl− are obvious, as are simple molecular ions such as [NO3]−.Things becomemore subtle when we consider metal coordination complex ionicspecies such as [AlCl4]−, which were used extensively during the early work onILs, as reviewed briefly in the following. These are certainly ionic as written, andas long as they continue to stay bound in the real liquid, for long times, then theyfit our definition. However, there is always an equilibrium process by which suchcomplex species are formed, and therefore we must always recognize the pres-ence, in equilibrium, of some amount of the component species. In the case of[AlCl4]−, this might be Al3+ and Cl−; as long as these components involved arealso ionic, we still have an IL, although with more complex ‘speciation’ than is atfirst apparent. This speciation, and how it responds to variables such as temper-ature, can influence the IL’s properties significantly.In the case of Co(H2O)62+, which sits in equilibrium with H2O and Co2+, the

situation is more complicated; [Co(H2O)6][NO3]2 is a simple hydrate salt thatmelts into a stable liquid at 55 ∘C.Where the other species are neutral (e.g., H2O),we have non-ionic components intruding into our IL and at some stage we mustbegin to think of the liquid as a mixture rather than a pure IL (recall that our def-inition includes the word ‘entirely’). It becomes important to consider where thedividing line lies between a ‘pure’ IL, meaning containing only the ions we thinkare present, and a ‘mixture’ of ions and other species. MacFarlane and Seddonproposed some time ago that a practical approach to this problem be adoptedbased on the fact that very few of any of our laboratory chemicals are absolutelypure – in fact, 100% purity is a practical impossibility. So, in reality, wemay acceptsomething as ‘pure’ if it is at least 95% of the desired compound– or perhaps 99%for high-quality/sensitivity work. Another perspective, which underlies much ofour laboratory chemistry, is that we consider something to be pure if the levelof contaminants (often starting with water and the ever-present dissolved N2andO2) is not sufficient to significantly affect its properties; this requires thought,


judgment, and discussion of what is ‘significant’. This, quite reasonable, practicalapproach avoids pedantic arguments about whether something does or does notfit our definition of an IL. This also allows our definition to cover a host of veryinteresting and important complex-ion species.Therefore, to summarize, throughout this book we will simply refer to an IL

as being any ‘liquid that is comprised entirely of ions’, with the word ‘entirely’being used in a practical rather than absolute sense. Although our definitionnow includes the classical field of molten salts, we will focus mostly on theambient-temperature ILs that have become of immense interest in the last20 years.Other types of liquid systems that may be considered related to ILs but fall

outside of our definition, and are not covered here, are liquid mixtures of ionswith molecular species or ions with water. These complex liquid systems havebeen discussed in a number of recent reviews and books [3–7].

1.3 A Brief Perspective

In this book we will discuss many of the different types of ILs now available andthe applications being developed. However, it is also important to appreciate theorigins of this field and the path through different ion chemistries that has ledus this point. As our definition above highlights, it is a mistake to put ILs andhigh-temperature molten salts into two separate classifications and treat them asdifferent fields. While this book primarily concentrates on those species that areair- and water-stable and liquid at room temperature, because those are the focusof the majority of present research reports, other salt families also have much toteach us [8, 9].Thefirst ‘discovery’ of a room-temperature IL is often attributed to PaulWalden

whomade ethyl ammoniumnitrate, which is a protic ionic liquid (a class of IL dis-cussed further below) [10]. He recognized that the low melting point (13–14 ∘C)was a result of the larger organic cation that decreases the degree of ion associa-tion compared to an inorganic salt. It is also likely that liquid organic salts wererecognized by organic chemists with annoyance long before Walden, when theyprobably described them as ‘intractable oils’ and disposed of them.Remarkably, the ability of ammonium salts to dissolve cellulose was first

recognized in 1934 [8, 11]; the use of ILs for processing biomass (which com-monly includes cellulose) is now an important and extensive area of investigation,discussed in more detail in Chapter 6.Angell and his group began to investigate ambient-temperature systems in the

1970s [12], and then in the 1980s chloroaluminate-based salts were developedthat combined pyridinium or imidazolium cations with the tetrachloroaluminateanion ([AlCl4]−), again demonstrating the value of a large organic cation in reduc-ing the melting point [9, 13, 14]. Depending on the composition, some of thesesalts are liquid at room temperature and primarily found application in batteriesandmetal electrodeposition.However, the composition of thesematerials is quitecomplex as, depending on the relative concentrations of the two components,they can also form multivalent species such as [Al2Cl7]− and have either (Lewis)

1.3 A Brief Perspective 7

acidic, neutral, or basic properties. These ILs (and their starting materials) arealso very moisture sensitive, with [AlCl4]− hydrolyzing to form corrosive HCl,which severely limits their application.One of the most significant advances in the evolution of ILs came with the dis-

covery of air- and water-stable species in 1992 [15] byWilkes and his group.Thisreport combined the 1-ethyl-3-methylimidazolium cation ([C2mim]+, which isstill one of the most widely used cations today) with [BF4]−, [NO3]−, [NO2]−,and acetate anions. We now know that some fluorinated anions (e.g., [BF4]− and[PF6]−) are in fact susceptible to hydrolysis [16], particularly at increased tem-peratures, and produce HF. Nonetheless, the significantly improved stability thatthese anions impart compared to the chloroaluminates is highly beneficial.From this point in time, the field of ILs expanded rapidly, both in terms of

the different ions used and in the range of applications being investigated. Theseapplications include those on a research lab-scale and also an industrial scale;the commercial use of ILs has been under development since the late 1990s [8].Table 1.1 shows the structures and common abbreviations of the most widelyused IL ions. It can be seen that there are a variety of different fluorinated anionsin use because these commonly produce low melting points, relatively low vis-cosities, and good electrochemical stability.This is primarily a result of the chargedelocalization over the anion (because F is so electronegative). The relationshipbetween the chemical structure of ILs and the different physical properties (vis-cosity, melting point, etc.) is discussed throughout this book. The [NTf2]− anionis a classic example of a fluorinated anion [17, 18], and for the same reasons asthis is attractive for IL electrolytes, it has also been widely used in the batterycommunity for many years (originally just as the lithium salt, Li[NTf2]).The cations used to make ILs are still predominantly nitrogen-based, although

phosphonium cations are becoming increasingly common and can impart bet-ter stability [19, 20]. Methods for synthesizing common nitrogen-based ILs arediscussed in Chapter 4. Another emerging family of ILs comprises ‘solvate ionicliquids’, which are concentratedmixtures of salts and a solvent –most commonlylithium salts and oligoether (also known as ‘glyme’) solvents [21, 22]. The multi-ple oxygens on the glyme strongly solvate the lithium ion, essentially forminga [Li(glyme)]+ cation that is balanced by the counter-ion from the lithium salt(most commonly [NTf2]−). Thus, they have a relatively ionic nature and simi-lar behavior to ILs (e.g., low vapor pressure) despite the presence of a molecularcomponent. Not all concentrated mixtures of lithium salt and solvent form ‘sol-vated ILs’, that is, if the coordination of the Li+ is not strong enough. In thatcase, the physical properties of the mixture would not be consistent with a purecation/anion combination and would therefore fall outside our definition of anIL. However, many concentrated mixtures do form ‘solvated ILs’ and these showgreat promise in various battery applications [21, 22].While much of the early work on ILs/molten salts was driven by interest

in their use as electrolytes, the potential benefit of these materials to ‘greenchemistry’ – on a lab scale and in industry – has also been an important drivingforce. However, ILs should not be claimed as ‘green’ solvents simply because theyhave negligible vapor pressure (and thus do not emit the toxic vapors of manyvolatile organic solvents). Factors such as biodegradation, toxicity, and recyclingof the IL must also be considered. The ‘12 Principles of Green Chemistry’


[23] also include parameters such as reducing the number of reaction steps,minimizing overall energy consumption, and decreasing the amount of materialsused, which are as important for designing green engineering processes; allof these must be considered before classifying the use of an IL as ‘green’. Thetoxicity and biodegradation of ILs is discussed further in Chapter 9.Finally, one other frequently proclaimed feature of ILs is their ‘tunability’. Once

we understand that we can use large charge-diffuse ions to decrease the meltingpoint of high-temperature molten salts, then we realize that there are an extraor-dinarily large number of ions that we can choose from to make our IL.Therefore,if we know which properties the different ions will produce, we can ‘tune’ the ILto suit our application (e.g., choosing a fluorinated anion to increase the electro-chemical stability or decrease miscibility with water). The phrase ‘task-specific’ionic liquid is sometimes used, which originally referred to the use of function-alized alkyl groups to improve the partitioning of specific metal ions into the ILphase from water [24], but this terminology has since expanded in scope. Theuse of IL mixtures to achieve the desired properties allows a further dimensionin tunability, and this is discussed further in Chapter 5. In general, however, weare still some way from being able to design a cation/anion combination fromfirst principles to optimize a specific physical property. The reasons for this willlikely become clearer as we progress through this book and the complexity ofthe relationships between the chemical and the physical properties of ILs is dis-cussed further. In the future, computational techniques for modeling ILs, whichare introduced briefly in the following and in Chapter 5, are likely to bring signif-icant benefit toward ‘designing ILs to suit the task’.

1.4 Aprotic Versus Protic ILs

Thus far we have discussed ions that do not contain any labile (or transferable)protons; these are called ‘aprotic’ ILs meaning ‘not protic’. On the other hand, ILscan actually be formedby proton transfer fromaBronsted acid to aBronsted base,and these are termed ‘protic ionic liquids’ (PILs). This family has some uniquecharacteristics that need additional consideration with respect to how they fitwithin our definition of an IL.The physical properties of PILs can depend very strongly on the relative

strengths of the acid and base, as this determines the extent of proton transfer toform the salt:

HA + B−−−−⇀↽−−−−[BH+][A−] (1.1)

The free-energy change associated with the proton transfer process, as well asthe relationship between this and ΔpK a (the difference between the pK a valuesof the acid and base), has a direct influence on properties of the liquid such as itsvapor pressure, which in this case is nonzero at most temperatures, and its ‘ion-icity’ (as discussed further in Chapter 5). This is because the free-energy changedetermines the extent of proton transfer and thus the amount of free acid and basepresent in the PIL. It was this problem that prompted Seddon and MacFarlane

1.5 An Overview of IL Applications 9

to propose the 95% (or 99%) guideline discussed above as a practical definitionof when these could be called ILs. The presence of these neutral species affectsthe melting point (one could think of these as impurities within the PIL). Also, itcan be difficult to remove all traces of water from PILs, as extended drying undervacuumwill remove the volatile acid or base components, shifting the above equi-librium and changing the composition.It is important to note that the acid/base stoichiometry in a PIL does not have to

be 1 : 1; in fact, it is quite hard to achieve genuinely 1 : 1 stoichiometry.The actualcomposition can impact significantly the chemical and thermal properties of theliquid [25, 26]. Such off-stoichiometry compositions must also be considered tobe mixtures in most cases – the melting behavior of mixtures is also discussedfurther in Chapter 5.

1.5 An Overview of IL Applications

When considering all the different applications of ILs, of which there is anever-growing number, it is useful to relate these back to their unique properties.We often introduce ILs as having properties that can include ‘low meltingpoint, negligible vapor pressure, good electrochemical and thermal stability,and tunable structures’, and so forth, although it is very important in such anintroduction to note that not all ILs have all of these properties. So how dothese characteristics translate to application, and which properties still need themost investigation and improvement? The following provides some examples ofparticular properties attracting certain applications.The lowmelting point of ILs drives interest in their use as pharmaceutical salts,

where the cation or anion is an active pharmaceutical ingredient [27–29]. Thelow melting point removes the concern of a salt crystallizing into an alternativepolymorph (crystal structure) from that which has been trialed and patented,as the formation of polymorphs has significant medical and legal implications.Having the drug in a liquid form may also make it easier to be administered topatients. This application of ILs is discussed further in Chapter 9.The ionic nature of ILs also means that they provide quite unique solvation

environments compared to conventionalmolecular solvents, and this is exploitedin a variety of different synthetic reactions, materials processing/extraction, andgas separation (as discussed further in Chapter 6).There is also extensive interestin their use for biomass processing. Biomass is a complex mixture of materi-als that can include cellulose, hemicellulose, keratin, lignin, chitin, and so on,depending on its source, and it is a valuable sustainable resource. However, thesematerial mixtures are notoriously insoluble in molecular solvents, which makestheir chemical processing difficult. ILs are being investigated for both the disso-lution of a variety of different biomaterials and for their processing into highervalue products [29, 30]. For biotechnological applications, the ability of ILs todissolve and stabilize enzymes and proteins, DNA, and RNA is also extremelyvaluable. Note, however, that in these applications often the IL must contain asmall amount of water, and therefore they are sometimes referred to as ‘hydratedILs’. These applications are discussed further in Chapter 9.


Unique solubilizing properties, coupled with good electrochemical stability,also underlie the use of ILs for rare-earth processing and recycling [31].Rare-earth metals are used in significant quantities globally as they haveunique magnetic, luminescent, and electrochemical properties, but supplies areincreasingly becoming limited. ILs are promising for two techniques – as anionic extractant for the separation of rare-earth salts, and as the medium for thesubsequent electrodeposition of the pure rare-earth metal. The electrodeposi-tion of rare-earth metals requires very electrochemically stable, aprotic media,and until recently this process has primarily utilized high-temperature moltensalts. The use of ILs for the electrodeposition of a variety of metals is discussedin Chapter 7.Excellent electrochemical stability is arguably one of the most important

characteristics of some ILs, as evidenced by their extensive use in electrochemi-cal devices, for electrowinning, water splitting, and so on [32–34].Water splittingdescribes the electrolysis of water into hydrogen and oxygen. This is of greatinterest for the large-scale production of hydrogen as a fuel, particularly if theenergy for the electrolysis can be provided by the sun (i.e., photoelectrochemicalwater splitting). Central to this process is an efficient electrocatalyst for the wateroxidation reaction, as it is the kinetics of this reaction that is the more limitingfactor. ILs have been used as a medium for the synthesis of good water-splittingcatalysts such as MnOx, where x is typically between 1.5 and 2 [2]. Althoughthese deposition and oxidation processes are still under investigation, one ofthe key properties of the IL here appears to be the structure that it impartsto the liquid phase (and the liquid/electrode interphase) during the reaction,which influences the thermodynamics of the process. The use of ILs to enhancethe water oxidation reaction (i.e., decrease the overpotential) is also central totheir use in metal–air batteries. The use of ILs in electrochemical processes anddevices is discussed in Chapters 7 and 8.In addition to the synthesis of water-splitting catalysts mentioned previ-

ously, ILs have, of course, found application in a wide range of other syntheticreactions – organic, inorganic, biological, and so on. Again, related to the uniqueproperties of ILs, the benefits of their use are multifold. Most simply, the abilityto dissolve materials that are insoluble in common organic solvents is highlyadvantageous, while their ionic nature helps in stabilizing nanoparticle disper-sions during synthesis. At a more sophisticated level, the term ‘ionothermalsynthesis’ describes the use of ILs as both the solvent and as a structure-directingtool [29, 35].This has been widely utilized for the synthesis of a range of valuablematerials including molecular sieves, metal–organic frameworks (MOFs),reduced graphene oxides, and polyoxometalates, during which the nature ofthe IL used can impact the resultant material structure. In this application, thelow vapor pressure and good thermal stability of ILs also provide an importantadvantage: such reactions use temperatures up to 200 ∘C, and therefore ifwater is used (i.e., in ‘hydrothermal synthesis’), this requires the reaction tobe performed in an autoclave. In contrast, the ionothermal reaction can oftenbe carried out in ordinary equipment. However, while the templating effectsof ILs for the synthesis of these materials are intriguing, full understandingor control of the self-assembly processes that underlie such synthesis requires

1.5 An Overview of IL Applications 11

further investigation. The nature of the IL and their structure (in the liquidphase and at solid/liquid interfaces) also influence the synthesis of a variety ofnanoparticles [36]. The structural ordering of ILs, as well as its impact on somesynthetic processes, is discussed in Chapter 3.Other synthetic applications of ILs include the use of chiral ILs in stereos-

elective synthesis, where they can be used as the catalyst or as the syntheticmedium [37]. Here, the most important and unique property of the IL is theability to incorporate a chirally pure moiety into the ionic structure, the sourceof which may be natural (e.g., amino acid-based ILs) or synthetic. Chiral ILs alsohave application in chromatography and spectroscopy, for example, as NMRchiral shift reagents.In addition to chiral catalysis, ILs can also be used for heterogeneous, homo-

geneous, and biocatalysis (whole-cell and enzyme-catalyzed). This is a verywide-ranging application of ILs and is the focus of multiple books and reviews[38–40], and is discussed further in Chapter 6. In this application there aretwo important material classes: supported ILs, that is, the use of a porous solidsupport to immobilize an IL containing a dissolved homogeneous metal catalyst,and ‘solid catalyst with IL layer’, where a heterogeneous catalyst is coated withan IL to improve the properties [40]. In terms of the application of ILs in theliquid form, these are commonly used in a biphasic system with another solvent.In this application, the solubilizing properties of the IL are the key.Although it is not yet a large field, it is interesting to note the use of ILs as heat

transfer fluids, as this is an example of an application that benefits from the rel-atively high heat capacity and good thermal stability of ILs. For this application,low vapor pressures, viscosity, and corrosivity are also important requirements.Nanoparticles can also be added to the IL to improve the heat capacity and ther-mal conductivity [41, 42].In the field of energetic materials [43], the huge structural variability of ILs is

a great advantage, as are their low vapor pressure, wide liquid range, and goodthermal stability. Energetic materials are those with a large amount of storedchemical energy that can be released by, for example, shock, heating, or apply-ing friction, and are thus used as explosives, propellants, and so on. As one classof such materials, ‘hypergolic ILs’ refer to ILs designed to ignite when in con-tact with a suitable oxidizer. These ILs are being designed as a replacement forhydrazine, which is highly toxic and difficult to handle, for use in propellants.Nitrogen-containing heterocyclic cations (e.g., substituted alkyl ammonium, imi-dazolium, or triazolium cations) are commonly combined with anions such asnitrate, dicyanamide, nitrocyanamide, cyanoborate, azide, or aluminum borohy-dride (which contain energetic groups –NO2, –N3, –CN, etc.). Although thesesalts are often solid at room temperature, and even above 100 ∘C, they still fitwithin our definition of ILs and are the subject of a significant number of researchefforts.The use of ILs in tribology, that is, as lubricants to reduce the wear and friction

between two moving parts, benefits primarily from their low vapor pressure,good thermal stability, synthetic versatility, and the ability to interact stronglywith different surfaces. This includes, in some cases, forming surface films thatare composed of IL degradation products. Another possible advantage is the


ability to control the viscosity, which is an important determinant of lubricationperformance, by using IL mixtures. There is a very large potential commercialmarket for this particular IL application. Cations with long alkyl chains arefrequently used, as these have structural similarity with the common baseoils, typically with fluorine- and phosphorous-containing anions, for example,[NTf2]− or diphenyl phosphate [44, 45]. ILs can also be mixed with the baseoils that are currently used; the IL thus replaces the normal additives. This alsoreduces the cost associated with the IL, as less quantity is needed while stillmaintaining the efficacy of lubrication [46]. However, achieving miscibility ofthe IL with these relatively nonpolar materials can be a challenge.Finally, another application for ILs with significant commercial potential

is their use in sensors. Here we are primarily referring to electrochemicalsensors, where the wide electrochemical window and nonvolatility of ILs are aconsiderable advantage. Often, for this application the IL is used to modify anelectrode surface (e.g., glassy carbon, platinum, or graphite) or used as a layer ona quartz crystal microbalance. Two important uses of IL-based sensors are thedetection and quantification of gases and of biological species. For both chemicaland biological sensing, the ability to optimize the solubility of the target analyteby tuning the nature of the IL ions is very important [47, 48].

1.6 Key Properties and Techniquesfor Understanding ILs

The above discussion gives an indication of how different applications benefitfrom the properties of ILs. To further develop such applications, it is important tofully understand the origin and unique characteristics of ILs.This understandingwill then allow us to design anions and cations that impart further improvementsin IL properties and performance. In this section we introduce the key proper-ties of ILs that we are commonly most interested in and the techniques that areavailable for probing them.

1.6.1 Viscosity

One of the holy grails of IL research is achieving lower viscosities, as thiswould enable further improvements in the performance of electrochemicaldevices through improved ion transport, make lab-scale use easier, simplify thedesign of fluid handling for commercial processors, and so on. The viscosityof ILs is relatively high compared to other solvents – at least 10 times higherthan water and often significantly greater (think of the consistency of oil oreven honey). This is a result of the strong intermolecular interactions: vander Waals forces, hydrogen bonding, and Coulombic forces. The viscosity ofneat ILs and their mixtures is discussed further in Chapter 5. Mixing ILs withmolecular solvents is a very effective way of reducing the viscosity, but thisgenerally comes at the expense of an increase in vapor pressure, flammability,lower electrochemical stability, and so on. As with many of the other propertiesof ILs, experimental measurement of the viscosity requires a clean and dry

1.6 Key Properties and Techniques for Understanding ILs 13

IL and measurement under an inert atmosphere. This should be taken intoconsideration when selecting the viscometer to be used, as should be the samplesize required (which, for ILs, we normally want to be as small as possible) andthe Newtonian or non-Newtonian behavior of ILs.[49]. If the IL behaves as aNewtonian fluid, that is, the viscosity does not change with sheer rate, then theviscosity can be accurately measured by rolling-ball, capillary, or rotational (e.g.,cone and plate) viscometers. To assess the non-Newtonian viscosity of an IL (orof an IL containing an additional dissolved or suspended species as this oftenintroduces non-Newtonian behavior), a rotational viscometer is to be used.Further, magnetic resonance velocity imaging is a recently reported techniquethat allows magnetic resonance images to be collected within the cone and plateviscometer [50]. This allows further information about the rheological behaviorof ILs with and without suspended nanoparticles to be obtained.The rheologicalbehavior of ILs may also be related back to the effect of ordering of the liquidstructure of ILs [51, 52], which is discussed further in Chapters 3 and 5. Finally,it is useful to note that the viscosity of ILs decreases relatively rapidly withtemperature, and this effect can be utilized for ease of handling in the lab or inlarger scale commercial reactor design.

1.6.2 Vapor Pressure

Negligible vapor pressure is arguably the physical property of ILsmost commonlyreferred to as beneficial for application but one of the least investigated. Forexample, how does this depend on the nature of the IL and what exactly do wemean by ‘negligible’? (Note that we should never say ‘no vapor pressure,’ as thisis a thermodynamic impossibility, except at absolute zero kelvin). Contrary tothe initial understanding in the field, Earle and coworkers famously showedthat some aprotic ILs are, in fact, distillable and therefore must have a signif-icant vapor pressure [53]. One technique for quantifying this as a function oftemperature is a combination of the Knudsen effusion apparatus and a quartzcrystal microbalance [54, 55]. Note that our discussion of vapor pressure ofaprotic ILs is fundamentally of different origin from the vaporization of PILs,which occurs by evaporation of the constituent acid and base (see Eq. 1.1) asneutral molecules into the gas phase. Aprotic ILs are now believed to exist inthe gas phase as strongly associated ion pairs (Figure 1.1), which is supportedby both experimental and computational studies [56, 57]. Information aboutthe enthalpy of vaporization of ILs also helps us to understand the strengthof interactions between ions in the liquid phase. More comprehensive studiesof the vapor pressure of different ILs and an understanding of the gas-phasestructures are seen as important areas of future study.

1.6.3 Melting Point

The other often-reported property of ILs is the low melting point, the origin ofwhich is discussed in Chapter 2. However, we still have to learn much before wecan quantitatively understand how different ions affect this property and howwe can therefore design new low-melting species. The most common techniquefor measuring the melting point is differential scanning calorimetry (DSC).


B

B

BB

X–

X–

X–

X–

X–

X–

X–

X– X–

X–X–

X–X–

X–

X–

X–

X–

X– X–

X–

X–

X–

X–X– X–

X–

X–

(BH)+

(BH)+

(BH)+

(BH)+

(BH)+

(BH)+

(BH)+

(BH)+

(BH)+(BH)+

(BH)+(BH)+

(BH)+

BX–

X–

X–

A+

A+

A+

A+

A+

A+A+ A+

A+

A+

A+ A+

A+

A+

A+A+A+

HXHX

HX

HX

HX

Protic ionic liquid Aprotic ionic liquid

Figure 1.1 Illustration of the difference between the volatilization of PILs, where neutralspecies evaporate into the gas phase and aprotic ILs that exist as tightly bound ion pairs in thegas phase. (Adapted from Earle et al. 2006 [53]. Reproduced with permission of NaturePublishing Group.)

Measurement of melting point can also be complicated by the tendency of manyILs to supercool and form glasses, which is another property that is significantlyaffected by impurities in the IL.

1.6.4 Nanostructure

A more complex characteristic of ILs is the formation of nanometer-levelstructure in the liquid phase. This is not commonly described as an advantagebut is certainly quite special and can have a significant influence on the propertiesand behavior of ILs (and reactions in ILs). As discussed in detail in Chapter 3,this is still a young area of study, in which techniques such as X-ray and neutrondiffraction are predominantly used to reveal the nanometer-level detail [58].The structure of ILs at surfaces, including at solid surfaces such as inorganicnanoparticles in the IL, and at interfaces (liquid/liquid or liquid/gas) is evenmore complex but equally important, as it impacts the behavior of ILs in manyapplications.

1.6.5 Thermal Properties

There is a range of thermal properties that are important for IL applications.Mostimportant and most widely referenced is their thermal stability; unfortunately,this is often overestimated. Commonly, thermal gravimetric analysis (TGA) isused, where the IL is heated at a controlled rate and the decrease in mass of thesample analyzed. The onset of decomposition is then taken from the intersec-tion of the initial baseline with the tangent of the plot over the region of mostrapid weight loss. However, weight loss of the IL will have occurred before this


‘onset’ point; it has simply not been allowed sufficient time to become significantat the heating rate used, because the experiment is normally relatively fast (e.g.,10 ∘Cmin−1).More useful data is themaximumrecommended operating temper-ature of that IL belowwhich degradation is negligible, quantification of the rate ofdecomposition, and an understanding of the decomposition products [59]. Thislatter information can be obtained by a TGA coupled to a mass spectrometer orby using pyrolysis mass spectrometry. This is discussed in Chapter 5.The thermal conductivity of ILs is of great importance to their application as,

for example, heat transfer fluids. However, there has been little experimentalmeasurement or theoretical prediction of this property [60]. The most commonexperimental approaches to this measurement involve either placing the ILbetween two parallel plates, one of which is then heated, or inserting a heatingwire into the sample. The thermal conductivity is then calculated from eitherthe heat flow from the upper plate through the IL to the cooling plate, or theheating rate of the wire.The appropriate mathematical relationship must then beused to take into account the setup geometry used and separate out the thermalconductivity and thermal diffusivity of the IL.The heat capacity of ILs (the quantity of heat required to raise the temper-

ature by 1∘C) is also crucial for their application as heat transfer fluids and isanother property not widely studied [61, 62], or predicted [63]. DSC and modu-lated DSC are the most commonmethods of measurement, although other typesof calorimetry can also be used. The heat capacity generally increases linearlywith the molar mass of the IL, but for mixtures of ILs this becomes more com-plex. The heat capacity and thermal conductivity of ILs are discussed further inChapter 5.In many applications, such as the use of ILs in batteries or synthesis, it is very

important to know about the potential for ‘thermal runaway’ during heating,which can cause explosions. If the heat produced in a reaction or, for example,at an electrode surface is greater than the capacity for the IL to remove thatheat, then the system will self-heat and thermal runaway can occur. The likeli-hood of this happening for different ILs can be measured by accelerating ratecalorimetry (ARC).This is a seldom-used technique but one that provides crucialinformation regarding the safe use of ILs. In this method, the IL is heated in smalltemperature steps, and at each step the instrument detects whether an exother-mic process is occurring, that is, heat is being released, as opposed to the IL beingable to stabilize at that temperature. In one study [64], for a series of [C2mim]+ILs, no self-heating was observed with the [BF4]−, [NTf2]−, or diethylphosphateanions, which is the behavior desired for their use as solvents or electrolytes.However, ILs with the [fsi]−, [B(CN)4]−, and tricyanomethanide anions exhib-ited exothermic self-heating behavior and an accompanying pressure increase,with onset temperatures of 165, 316, and 230 ∘C, respectively. What this means,in a practical sense, is that, if these ILs are exposed to temperatures in excess ofthese ‘onset temperatures,’ then extreme temperature and pressure rises couldspontaneously occur and, for example, sealed battery packs may burst open.ThisARC technique can also be used to assess the thermal runaway characteristicsof IL electrolytes containing lithium salts and common Li-ion battery electrode


materials and compare the behavior to that of the commercially used carbonateelectrolytes. This has confirmed [C2mim][fsi] to be the worst IL among thosetested, and indicates that not all ILs are better (in this regard) than the carbonatesolvents [65].

1.6.6 Electrochemical Properties

Finally, it is primarily the electrochemical properties of ILs that enable theirextensive use in batteries, supercapacitors, fuel cells, solar cells, electrowinning,and so on. By ‘electrochemical properties’ we are predominantly referring togood electrochemical stability and relatively high ionic conductivity.The specificbehavior of ILs in batteries, their interfacial stability, and the formation of a pas-sivating ‘solid electrolyte interface’ are discussed in Chapter 8. Characterizationof the electrochemical stability of ILs is usually performed using cyclic voltam-metry, as discussed in detail in Chapter 7. Here, in terms of the thermodynamicsof the process, it is the structure of the cation and the anion that defines theoxidative and reductive stability of the IL: the most positive or negative potentialthat the IL can be exposed to without an electron-transfer reaction occurring,resulting in reduction of the cation or oxidation of the anion. However, thekinetics of these reactions and, thus, the behavior of the IL in actual applicationsdepend on factors such as the nature and morphology of the electrode surface,the strength of interactions at the IL/surface, the structure of this surface layer(and how far these layers penetrate into the bulk IL), temperature, IL viscosity,and conductivity.The arrangement of an IL at an electrode surface is defined by the ‘electric dou-

ble layer’, which describes how the potential decreases with the distance fromthe electrode surface. It is this phenomenon that forms the basis of double-layercapacitor operation, but understanding the potential profile across ILs is alsovery important for other electrochemical devices. Analyzing the structure of ILsat electrode surfaces (and how this changes with the potential of the surface) isextremely complex. Ideally, such analysis would be performed in situ (i.e., allow-ing us to look at the structure of an IL within an actual battery, fuel cell, etc.), butusing ex situ experiments such as atomic force microscopy (AFM) to gain a pic-ture of the arrangements of ILs on different solid surfaces (charged or uncharged)can also be extremely informative [58, 66].

1.6.7 Conductivity and Ion Transport

Theperformance of ILs in an electrochemical device is influenced not only by thestructure but also, very importantly, by the dynamics of the ions. Achieving a highrate of transport of the electroactive species to and from the electrode surfaceis crucial, and this is where ionic conductivity, diffusion rate of the electroac-tive species, and the transport number become relevant. The ionic conductivity,measured by electrochemical impedance spectroscopy (EIS), tells us about theaverage transport of all charged species – the IL, plus any other dissolved elec-troactive ions such as lithium salts. To obtain more species-specific transportinformation, cyclic voltammetry (CV)-based techniques can be used to mea-sure the diffusion rate of just the electroactive species through the solution [67].


This parameter is primarily affected by the viscosity of the IL. CV analysis isalso used to assess the metal deposition/stripping behavior of ILs containing, forexample, dissolved Li or Na salts. Measurement of the transport number (thefraction of charge carried by, for example, the Li+ ion in an IL/Li salt battery elec-trolyte solution) can be calculated from the steady-state current produced afterholding a potential across the electrolyte for an extended period [68].It is also interesting to note that the conductivity of ILs can tell us about

the extent of ion pairing or aggregation in the liquid. It is a result of this ionaggregation that the conductivity of ILs is often not nearly as high as we mightexpect if we were to simply consider the high concentration of ions – if theseions were all free to move independently, the ionic conductivity would be muchgreater than we actually observe.We can also compare the conductivity obtainedfrom standard electrochemical measurements (which measure the migrationrate of species in an applied electric field and thus is applicable only to chargedspecies) with that predicted fromNMR diffusion measurements (which measurethe diffusion of all species, charged or not). If we use the Nernst–Einsteinequation to predict the conductivity based on the diffusion data from NMRexperiments, then the difference between this value and the actual conductivity(which is always lower) tells us about the extent of ion aggregation in the IL [69].This ion aggregation parameter is commonly referred to as the ‘ionicity’ and isdiscussed in detail in Chapter 5.

1.6.8 Computational Techniques

There are many theoretical approaches that can help us to understand thechemical and physical properties of ILs. This field has benefited enormouslyfrom improvements in program sophistication and computer power (which isnecessary for the study of large systems) since the first computational studiesof ILs [70]. The techniques used for understanding and predicting the physicalproperties of ILs can generally be grouped into five categories [71], with roughlyincreasing levels of complexity:• those that correlate the physical properties of an IL with molecular volume or

density,• those that correlate the molecular structure of ILs using ‘molecular descrip-

tors’ (electronic, geometric, etc.), with their properties, (e.g., the quantitativestructure–property relationship QSPR approach),

• molecular dynamics (MD) simulations [72], which allow, as the name suggests,investigation of themolecular/ionic structure and interactions in an IL and alsotheir dynamics (i.e., transport),

• ab initio theory [73], where quantum chemistry principles are used to studythe energetics and structure of the ions and clusters of ions from first principlesusing electronic wave functions, and

• a combination of the third and fourth techniques, termed ab initio MD [74].We will discuss the application of computational techniques to investigation

of the physical properties of ILs throughout the book, for example, for meltingpoint in Chapter 2, liquid-phase structure in Chapter 3, and transport propertiesin Chapter 5.


One of the key difficulties of this field is that no single technique is accurate forthe simulation of all short- and long-range interactions, for the incorporation ofall enthalpy and entropy factors, for accurate simulation of the force fields (theenergy functions used to describe the potential energy of the system), and forlooking at the dynamics of the IL. Thus there is always a trade-off between highaccuracy and the scale of simulation that is feasible, i.e., how many ions can bestudied within a given amount of expensive computer time and over how longa time period. It is therefore up to the researcher to select the most appropriatetechnique for their research while being aware of the associated limitations of thetechnique.What is also important is to relate the results of theoretical predictionsback to the results of any experimental measurements available.

1.7 NewMaterials Based on ILs

As the field has progressed, a variety of IL-related materials have been devel-oped, where the IL is a starting point but has been designed to exhibit solid-likemechanical properties for applications where this is desirable. In the following,we look briefly at some of these important material classes and give suggestionsfor further reading on these subjects.IL-based polymer electrolytes are arguably one of the most commercially

viable applications of ILs (at least in the near future). This is driven by a rapidlyexpanding battery market, for example, for large-scale stationary storage, elec-tric vehicles, support for domestic photovoltaics and portable electronics, andimproving the safety of these devices. Here we are predominantly referring toIL-based electrolytes incorporating Li+ salts, although next-generation batteriesbased on other ions, such as Na+, are under development. ILs can potentiallybe used to replace the flammable and volatile organic solvents currently used incommercial Li ion batteries, and also avoid the use of Li[PF6] that decomposes toproduce highly toxic HF. The use of ILs, in their liquid form, in electrochemicaldevices is discussed in Chapter 8. The benefit of using an IL within a polymericor otherwise solidified system is that it prevents leakage, for example, if thebattery is damaged, and provides a mechanical barrier to the growth of lithiumdendrites that can cause short-circuiting and fire. The most common class arethose formed by IL addition, effectively as a plasticizer, to a polymer/Li saltsystem such as PEO/Li[NTf2] [75]. When the IL becomes a major component,these materials are also commonly referred to as gels or ionogels. However, theextensive field of solid or gel IL-based electrolytes also encompasses polymerizedILs, zwitterionic gels (where the cation and anion are covalently tethered), thosesolidified using inorganic nanoparticles, and ILs with alkoxy-silane functionalgroups attached to the cation. Central to the development of all of these materialclasses for use in lithium batteries is achieving sufficient transport of thetarget Li+ ion. Low transport rates is one of the primary limitations of presentsolid-state IL-based battery electrolyte materials and the focus of an extensiveresearch effort [76, 77].The development and use of IL-based gels is not restricted to the battery

field – other types of IL gels have application in electrochemical devices (fuel

1.7 New Materials Based on ILs 19

cells, electrochromics, supercapacitors, etc.), in catalysis, and in separation andanalysis, for example, as the stationary phase for gas chromatography [78]. Thecombination of ILs and polymers is also widely used in applications such as CO2separation [79], polyelectrolyte membrane fuel cells [80, 81], and actuators [33].The concept of ‘IL-based gels’ encompasses a wide range of materials, includingtwo-phase heterogeneous mixtures of IL with polymers, ceramics, carbon-basedmaterials, metal nanoparticles, or macromolecules. Central to all of this researchis the desire to maintain or even improve the desirable properties of the IL,such as the wide electrochemical window, low volatility, and ion conduction,while also achieving the advantages of a solid-state material such as mechanicalstrength, reduced leakage, increased device stability, or even the incorporationof additional functionality such as catalysis [82].Polymerized ILs, or poly(ionic liquids), are another approach to combining the

advantageous properties of ILs and polymers [83, 84]. Analogous to many of theapplications of IL-based gels, these can be used as electrolytes in batteries, fuelcells, supercapacitors, and so on, as gas separation membranes, and also as solidsupports for catalytic materials [78].The synthesis of these materials is discussedin Chapter 4.A further class of solid-like IL-based material is obtained by impregnating

the IL within porous solid supports such as silica, activated carbons, porousglasses, or MOFs [85, 86]. This encompasses materials known as ‘supportedionic liquid phase’ (SILP) catalysts [87], which are based on thin films of ILs onhigh-surface-area solid supports: these are used to immobilize transition-metalcatalysts. The properties of the IL-impregnated material depend on the amountof IL on the surface: that is, whether it is present as a monolayer that may becovalently bound to the surface, or as a multilayer. Multilayer systems allow thedissolution of additional species, such as catalysts, acids, or nanoparticles. Theprimary applications of ILs coated on solid supports are catalysis, for example,using transition metals, enzymes, acids, or nanoparticles within the IL, andseparation technologies such as gas purification and CO2 separation. ILs on asurface, or confined into a small space [88], can behave quite differently from thebulk IL.Therefore, understanding the properties and structure of ILs on surfacesis crucial for developing their application [58, 89].ILs on solid supports, such as silica columns, can also be used as stationary

phases for chromatographic applications [90–92]. Gas and liquid chromatogra-phy both benefit from the low vapor pressure and thermal stability of ILs, whilethe nature of the cation and anion can be used to adjust the retention behavior ofthe column and even allow separation of both polar and nonpolar analytes. Capil-lary electrophoresis, where the IL is again supported on a column, andmass spec-troscopy techniques such asmatrix-assisted laser desorption/ionizationMALDI,where the IL provides the matrix, are other analytical applications.Finally, in our discussion of porous supports, it is interesting to note that ILs

are also good precursors for the synthesis of carbon-based materials. Heatingthe IL (protic or aprotic, or polymerized ILs) at high temperatures results in car-bonization. This can produce carbon-based materials with well-controlled mor-phology and heteroatom doping, such as the incorporation of N atoms within thecarbon structure. The heteroatoms are very important for achieving the desired


catalytic properties for the carbon structures. ILs with cross-linkable groups,such as nitriles, are particularly good for this application, as cross-linking occursduring heating and helps in producing well-controlled morphologies and a moreeven distribution of the N dopants [32, 93, 94].

1.8 Nomenclature and Abbreviations

The full names of many ILs are very long and unwieldy, so a range of more simpli-fied names and acronyms are commonly used. Table 1.1 summarizes the commonnames, structures, and recommended abbreviations that are used in this book.Following recommended inorganic nomenclature, acronyms such as [dca]− arein lower case letters in square brackets. Charges are shown on isolated ions butnot when part of an ion pair. Atomic ions are shown without brackets.

References

1 Girard, G.M.A., Hilder, M., Zhu, H. et al. (2015) Electrochemical and physic-ochemical properties of small phosphonium cation ionic liquid electrolyteswith high lithium salt content. Phys. Chem. Chem. Phys., 17 (14), 8706–8713.

2 Zhou, F., Izgorodin, A., Hocking, R.K. et al. (2012) Electrodeposited MnOxfilms from ionic liquid for electrocatalytic water oxidation. Adv. EnergyMater., 2 (8), 1013–1021.

3 Greaves, T.L. and Drummond, C.J. (2015) Protic ionic liquids: evolvingstructure-property relationships and expanding applications. Chem. Rev.(Washington, DC, U. S.), 115 (20), 11379–11448.

4 Kohno, Y. and Ohno, H. (2012) Ionic liquid/water mixtures: from hostility toconciliation. Chem. Commun. (Cambridge, U. K.), 48 (57), 7119–7130.

5 Zhang, S., Wang, J., Lu, X., and Zhou, Q. (2014) Structures and interactionsof ionic liquids. Struct. Bond., 151, 1–201.

6 Reddy, P.N., Padmaja, P., Subba Reddy, B.V., and Rambabu, G. (2015) Ionicliquid/water mixture promoted organic transformations. RSC Adv., 5 (63),51035–51054.

7 Varela, L.M., Mendez-Morales, T., Carrete, J. et al. (2015) Solvation of molec-ular cosolvents and inorganic salts in ionic liquids: a review of moleculardynamics simulations. J. Mol. Liq., 210(Part_B), 178–188.

8 Plechkova, N.V. and Seddon, K.R. (2008) Applications of ionic liquids in thechemical industry. Chem. Soc. Rev., 37 (1), 123–150.

9 Wilkes, J.S. (2002) A short history of ionic liquids-from molten salts to neo-teric solvents. Green Chem., 4 (2), 73–80.

10 Walden, P. (1914) Molecular weights and electrical conductivity of severalfused salts. Bull. Acad. Imper. Sci. St.-Petersbourg, 405–422.

11 Graenacher, C. (1934). Patent No 1,943,17612 Angell, C.A. (1971) Fused salts. Annu. Rev. Phys. Chem., 22 (1), 429–464.13 Wilkes, J.S., Levisky, J.A., Wilson, R.A., and Hussey, C.L. (1982) Dialkylim-

idazolium chloroaluminate melts: a new class of room-temperature ionic

References 21

liquids for electrochemistry, spectroscopy and synthesis. Inorg. Chem., 21 (3),1263–1264.

14 Chum, H.L., Koch, V.R., Miller, L.L., and Osteryoung, R.A. (1975) Electro-chemical scrutiny of organometallic iron complexes and hexamethylbenzenein a room temperature molten salt. J. Am. Chem. Soc., 97 (11), 3264–3265.

15 Wilkes, J.S. and Zaworotko, M.J. (1992) Air and water stable 1-ethyl-3-methylimidazolium based ionic liquids. J. Chem. Soc., Chem. Commun., (13),965–967.

16 Freire, M.G., Neves, C.M.S.S., Marrucho, I.M. et al. (2010) Hydrolysis oftetrafluoroborate and hexafluorophosphate counter ions in imidazolium-basedionic liquids. J. Phys. Chem. A, 114 (11), 3744–3749.

17 Bonhote, P., Dias, A.-P., Papageorgiou, N. et al. (1996) Hydrophobic,highly conductive ambient-temperature molten salts. Inorg. Chem., 35 (5),1168–1178.

18 Koch, V.R., Nanjundiah, C., Appetecchi, G.B., and Scrosati, B. (1995)The interfacial stability of li with two new solvent-free ionic liquids:1,2-dimethyl-3-propylimidazolium imide and methide. J. Electrochem. Soc.,142 (7), L116–L118.

19 Bradaric, C.J., Downard, A., Kennedy, C. et al. (2003) Phosphonium ionic liq-uids. Strem Chem., 20 (1), 2–11.

20 Fraser, K.J. and MacFarlane, D.R. (2009) Phosphonium-based ionic liquids: anoverview. Aust. J. Chem., 62 (4), 309–321.

21 Mandai, T., Yoshida, K., Ueno, K. et al. (2014) Criteria for solvate ionicliquids. Phys. Chem. Chem. Phys., 16 (19), 8761–8772.

22 Tsuzuki, S. and Watanabe, M. (2014) Chemistry and application of glyme-type lithium solvate ionic liquids. Electrochemistry (Tokyo, Jpn.), 82 (12),1079–1084.

23 Anastas, P.T. and Zimmerman, J.B. (2003) Peer reviewed: design through the12 principles of green engineering. Environ. Sci. Technol., 37 (5), 94A–101A.

24 Visser, A.E., Swatloski, R.P., Reichert, W.M. et al. (2001) Task-specific ionicliquids for the extraction of metal ions from aqueous solutions. Chem. Com-mun., (1), 135–136.

25 Yoshizawa, M., Xu, W., and Angell, C.A. (2003) Ionic liquids by proton trans-fer: vapor pressure, conductivity, and the relevance of ΔpKa from aqueoussolutions. J. Am. Chem. Soc., 125 (50), 15411–15419.

26 Stoimenovski, J., Dean, P.M., Izgorodina, E.I., and MacFarlane, D.R. (2012)Protic pharmaceutical ionic liquids and solids: aspects of protonics. FaradayDiscuss., 154(Ionic Liquids), 335–352.

27 Stoimenovski, J., MacFarlane, D.R., Bica, K., and Rogers, R.D. (2010) Crys-talline vs. ionic liquid salt forms of active pharmaceutical ingredients: aposition paper. Pharm. Res., 27 (4), 521–526.

28 Shamshina, J.L., Kelley, S.P., Gurau, G., and Rogers, R.D. (2015) Chemistry:develop ionic liquid drugs. Nature (London, U. K.), 528 (7581), 188–189.

29 Smiglak, M., Pringle, J.M., Lu, X. et al. (2014) Ionic liquids for energy, materi-als, and medicine. Chem. Commun., 50 (66), 9228–9250.

30 Wang, H., Gurau, G., and Rogers, R.D. (2014) Structures and Interactions ofIonic Liquids, Structure and bonding(Berlin, Ger.), vol. 151, pp. 79–105.


31 Zhang, Q., Hua, Y., Xu, C. et al. (2015) Non-haloaluminate ionic liquids forlow-temperature electrodeposition of rare-earth metals – a review. J. RareEarths, 33 (10), 1017–1025.

32 MacFarlane, D.R., Forsyth, M., Howlett, P.C. et al. (2016) Ionic liquids andtheir solid-state analogues as materials for energy generation and storage.Nat. Rev. Mater., 1, 15005.

33 MacFarlane, D.R., Tachikawa, N., Forsyth, M. et al. (2014) Energy applicationsof ionic liquids. Energy Environ. Sci., 7 (1), 232–250.

34 Abbott, A.P., Dalrymple, I., Endres, F., and Macfarlane, D.R. (2008) Elec-trodeposition from Ionic Liquids, Wiley-VCH Verlag GmbH & Co. KGaA,pp. 1–13.

35 Morris, R.E. (2009) Ionothermal synthesis-ionic liquids as functional solventsin the preparation of crystalline materials. Chem. Commun., (21), 2990–2998.

36 Kang, X., Sun, X., and Han, B. (2016) Synthesis of functional nanomaterials inionic liquids. Adv. Mater.(Weinheim, Ger.), 28 (6), 1011–1030.

37 Payagala, T. and Armstrong, D.W. (2012) Chiral ionic liquids: a compendiumof syntheses and applications (2005-2012). Chirality, 24 (1), 17–53.

38 Pârvulescu, V.I. and Hardacre, C. (2007) Catalysis in Ionic Liquids.Chem. Rev., 107 (6), 2615–2665.

39 Hallett, J.P. and Welton, T. (2011) Room-temperature ionic liquids: solventsfor synthesis and catalysis. 2. Chem. Rev. (Washington, DC, U. S.), 111 (5),3508–3576.

40 Steinrueck, H.-P. and Wasserscheid, P. (2015) Ionic liquids in catalysis. Catal.Lett., 145 (1), 380–397.

41 Fox, E.B., Visser, A.E., Bridges, N.J., and Amoroso, J.W. (2013) Thermophys-ical properties of nanoparticle-enhanced ionic liquids (NEILs) heat-transferfluids. Energy Fuels, 27 (6), 3385–3393.

42 Lopez-Gonzalez, D., Valverde, J.L., Sanchez, P., and Sanchez-Silva, L. (2013)Characterization of different heat transfer fluids and degradation study byusing a pilot plant device operating at real conditions. Energy (Oxford, U. K.),54, 240–250.

43 Zhang, Q. and Shreeve, J.N.M. (2014) Energetic ionic liquids as explosivesand propellant fuels: a new journey of ionic liquid chemistry. Chem. Rev.(Washington, DC, U. S.), 114 (20), 10527–10574.

44 Kondo, Y., Koyama, T., and Sasaki, S. (2013) Ionic Liquids New AspectsFuture, pp. 127–141.

45 Somers, A.E., Biddulph, S.M., Howlett, P.C. et al. (2012) A comparison ofphosphorus and fluorine containing IL lubricants for steel on aluminium.Phys. Chem. Chem. Phys., 14 (22), 8224–8231.

46 Li, H., Cooper, P.K., Somers, A.E. et al. (2014) Ionic liquid adsorption andnanotribology at the silica-oil interface: hundred-fold dilution in oil lubricatesas effectively as the pure ionic liquid. J. Phys. Chem. Lett., 5 (23), 4095–4099.

47 Rehman, A. and Zeng, X. (2012) Ionic liquids as green solvents and elec-trolytes for robust chemical sensor development. Acc. Chem. Res., 45 (10),1667–1677.

48 Baker, S.N., McCarty, T.A., Bright, F.V. et al. (2009) Ionic Liquids in ChemicalAnalysis, CRC Press, pp. 99–137.

References 23

49 Holbrey, J.D., Rogers, R.D., Mantz, R.A. et al. (2007) Ionic Liquids in Synthe-sis, Wiley-VCH Verlag GmbH & Co. KGaA, pp. 57–174.

50 Novak, J. and Britton, M.M. (2013) Magnetic resonance imaging of the rheol-ogy of ionic liquid colloidal suspensions. Soft Matter, 9 (9), 2730–2737.

51 Smith, J.A., Webber, G.B., Warr, G.G., and Atkin, R. (2013) Rheology of pro-tic ionic liquids and their mixtures. J. Phys. Chem. B, 117 (44), 13930–13935.

52 Hu, Z. and Margulis, C.J. (2007) Room-temperature ionic liquids: slowdynamics, viscosity, and the red edge effect. Acc. Chem. Res., 40 (11),1097–1105.

53 Earle, M.J., Esperanca, J., Gilea, M.A. et al. (2006) The distillation and volatil-ity of ionic liquids. Nature, 439, 831.

54 Santos, L.M.N.B.F., Lima, L.M.S.S., Lima, C.F.R.A.C. et al. (2011) New Knud-sen effusion apparatus with simultaneous gravimetric and quartz crystalmicrobalance mass loss detection. J. Chem. Thermodyn., 43 (6), 834–843.

55 Rocha, M.A.A., Ribeiro, F.M.S., Schröder, B. et al. (2014) Volatility study of[C1C1im][NTf2] and [C2C3im][NTf2] ionic liquids. J. Chem. Thermodyn., 68,317–321.

56 Neto, B.A.D., Meurer, E.C., Galaverna, R. et al. (2012) Vapors from ionicliquids: reconciling simulations with mass spectrometric data. J. Phys. Chem.Lett., 3 (23), 3435–3441.

57 Leal, J.P., Esperança, J.M.S.S., Minas da Piedade, M.E. et al. (2007) The natureof ionic liquids in the gas phase. J. Phys. Chem. A, 111 (28), 6176–6182.

58 Hayes, R., Warr, G.G., and Atkin, R. (2015) Structure and nanostructure inionic liquids. Chem. Rev., 115 (13), 6357–6426.

59 Baranyai, K.J., Deacon, G.B., MacFarlane, D.R. et al. (2004) Thermal degrada-tion of ionic liquids at elevated temperatures. Aust. J. Chem., 57 (2), 145–147.

60 Koller, T.M., Schmid, S.R., Sachnov, S.J. et al. (2014) Measurementand prediction of the thermal conductivity of tricyanomethanide- andtetracyanoborate-based imidazolium ionic liquids. Int. J. Thermophys., 35 (2),195–217.

61 Paulechka, Y.U. (2010) Heat capacity of room-temperature ionic liquids: acritical review. J. Phys. Chem. Ref. Data, 39 (3), 033108.

62 Zábranský, M., Kolská, Z., Ružicka, V., and Domalski, E.S. (2010) Heat capac-ity of liquids: critical review and recommended values. Supplement II. J. Phys.Chem. Ref. Data, 39 (1), 013103.

63 Preiss, U.P.R.M., Slattery, J.M., and Krossing, I. (2009) In silico prediction ofmolecular volumes, heat capacities, and temperature-dependent densities ofionic liquids. Ind. Eng. Chem. Res., 48 (4), 2290–2296.

64 Vijayaraghavan, R., Surianarayanan, M., Armel, V. et al. (2009) Exothermicand thermal runaway behaviour of some ionic liquids at elevated tempera-tures. Chem. Commun. (Cambridge, U. K.), (41), 6297–6299.

65 Wang, Y., Zaghib, K., Guerfi, A. et al. (2007) Accelerating rate calorimetrystudies of the reactions between ionic liquids and charged lithium ion batteryelectrode materials. Electrochim. Acta, 52 (22), 6346–6352.

66 Fedorov, M.V. and Kornyshev, A.A. (2014) Ionic liquids at electrified inter-faces. Chem. Rev. (Washington, DC, U. S.), 114 (5), 2978–3036.


67 Bard, A. and Faulkner, L. (2001) Electrochemical Methods: Fundamentals andApplications, John Wiley & Sons, Inc.

68 Bruce, P.G. and Vincent, C.A. (1987) Steady state current flow in solid binaryelectrolyte cells. J. Electroanal. Chem. Interfacial Electrochem., 225 (1–2),1–17.

69 Tokuda, H., Tsuzuki, S., Susan Md Abu Bin, H. et al. (2006) How ionic areroom-temperature ionic liquids? An indicator of the physicochemical proper-ties. J. Phys. Chem. B, 110 (39), 19593–19600.

70 Hanke, C.G., Price, S.L., and Lynden-Bell, R.M. (2001) Intermolecular poten-tials for simulations of liquid imidazolium salts. Mol. Phys., 99, 801.

71 Izgorodina, E.I. (2013) Ionic Liquids Uncoiled, pp. 181–230.72 Maginn, E.J. (2009) Molecular simulation of ionic liquids: current status and

future opportunities. J. Phys. Condens. Matter, 21 (37), 373101.73 Izgorodina, E.I. (2011) Towards large-scale, fully ab initio calculations of ionic

liquids. Phys. Chem. Chem. Phys., 13 (10), 4189–4207.74 Hunt, P.A. (2006) The simulation of imidazolium-based ionic liquids. Mol.

Simul., 32 (1), 1–10.75 Passerini, S., Montanino, M., and Appetecchi, G.B. (2013) Polymers for Energy

Storage and Conversion, pp. 53–101.76 Osada, I., de Vries, H., Scrosati, B., and Passerini, S. (2016) Ionic-liquid-based

polymer electrolytes for battery applications. Angew. Chem., Int. Ed., 55 (2),500–513.

77 Armand, M.B., Bruce, P.G., Forsyth, M. et al. (2011) Energy Materials, JohnWiley & Sons, Ltd, pp. 1–31.

78 Mecerreyes, D. (2015) Applications of ionic liquids in polymer science andtechnology, Springer-Verlag, Berlin Heidelberg.

79 Dai, Z., Noble, R.D., Gin, D.L. et al. (2016) Combination of ionic liquids withmembrane technology: a new approach for CO2 separation. J. Membr. Sci.,497, 1–20.

80 Diaz, M., Ortiz, A., and Ortiz, I. (2014) Progress in the use of ionic liquids aselectrolyte membranes in fuel cells. J. Membr. Sci., 469, 379–396.

81 Yasuda, T. and Watanabe, M. (2014) Supported Ionic Liquids, pp. 407–418.82 Marr, P.C. and Marr, A.C. (2016) Ionic liquid gel materials: applications in

green and sustainable chemistry. Green Chem., 18 (1), 105–128.83 Shaplov, A.S., Marcilla, R., and Mecerreyes, D. (2015) Recent advances in

innovative polymer electrolytes based on poly(ionic liquid)s. Electrochim.Acta, 175, 18–34.

84 Yuan, J., Mecerreyes, D., and Antonietti, M. (2013) Poly(ionic liquid)s: anupdate. Prog. Polym. Sci., 38 (7), 1009–1036.

85 Fujie, K. and Kitagawa, H. (2016) Ionic liquid transported into metal-organicframeworks. Coord. Chem. Rev., 307(Part_2), 382–390.

86 Schwieger, W., Selvam, T., Klumpp, M., and Hartmann, M. (2014) PorousInorganic Materials as Potential Supports for Ionic Liquids. Supported IonicLiquids, Wiley-VCH Verlag GmbH & Co. KGaA, pp. 37–74.

87 Steinrueck, H.P., Libuda, J., Wasserscheid, P. et al. (2011) Surface science andmodel catalysis with ionic liquid-modified materials. Adv. Mater. (Weinheim,Ger.), 23 (22-23), 2571–2587.

References 25

88 Singh, M.P., Singh, R.K., and Chandra, S. (2014) Ionic liquids confined inporous matrices: physicochemical properties and applications. Prog. MaterSci., 64, 73–120.

89 Schulz, P.S. (2014) Spectroscopy on Supported Ionic Liquids. Supported IonicLiquids, Wiley-VCH Verlag GmbH & Co. KGaA, pp. 177–190.

90 Sun, P. and Armstrong, D.W. (2010) Ionic liquids in analytical chemistry.Anal. Chim. Acta, 661 (1), 1–16.

91 Apryll, M.S. (2008) Ionic Liquids in Chemical Analysis, CRC Press, pp.167–183.

92 Jared, L.A. (2008) Ionic Liquids in Chemical Analysis, CRC Press, pp.139–165.

93 Zhang, P., Zhu, H., and Dai, S. (2015) Porous carbon supports: recentadvances with various morphologies and compositions. ChemCatChem,7 (18), 2788–2805.

94 Zhang, S., Dokko, K., and Watanabe, M. (2015) Carbon materialization ofionic liquids: from solvents to materials. Mater. Horiz., 2 (2), 168–197.

Documents

1 AnIntroductiontoIonicLiquids · While much of the early work on ILs/molten salts was driven by interest in their use as electrolytes, the potential beneﬁt of these materials to